Document DE-A-1131372 discloses a cooking vessel having rotary blades installed therein. The rotary blades are fixed to a vertical shaft which is passed through an opening to the bottom of the vessel and fixed to an upper magnetic coupling member arranged in a hollow formed at the bottom. The cooking vessel is suitable for working with a cooking hob having a support plate and a lower magnetic coupling member driven by a motor for rotating below the support plate in a position close to a lower surface thereof. When the cooking vessel is placed on the support plate of the cooking hob, the mentioned upper magnetic coupling member faces the support plate and is very close to an upper surface thereof, such that the lower magnetic coupling member is capable of magnetically transmitting torque imparted by the motor to the upper magnetic coupling member to rotate the rotary blades inside the cooking vessel.
One drawback is that in the event that the rotary blades are locked, for example by food contained in the vessel, the vessel would tend to rotate around on the hob due to the effect of the torque transmitted by said magnetic couplings, which would entail a risk of accident.
Given that the support plate is smooth and continuous it is only possible to place the cooking vessel approximately on a treatment area of the support plate below which the lower magnetic coupling member is located, such that the shafts of the lower and upper magnetic coupling members are generally off-center. This entails another drawback since the off-centering of the rotating shaft of the upper magnetic coupling member may lead to a malfunction of the rotary blades.
If the coefficient of friction between the outer surface of the metal bottom of the vessel and the upper surface of the support plate is low enough, the cooking vessel will tend to slide on the support plate until it centers itself in relation to the rotating shaft of the lower magnetic coupling member due to the effect of the torque transmitted by the lower magnetic coupling member to the upper magnetic coupling member when the drive motor is activated. Nevertheless, this entails yet another drawback since this self-centering sliding of the cooking vessel on the support plate, if it occurs, may cause screeching, may scratch the support plate and degrade the upper surface thereof in the treatment area in a long run.